Conventional gas adsorption methods and devices today employ either fixed or moving beds of adsorbent in processes separating sulfur dioxide from flue gases resulting from coal combustion in power plants. Such fixed or moving bed methods and devices are also used to remove hydrogen sulfide from sewage treatment plant off-gases and to remove toxic metals from smelter gases.
The use of a fixed bed is the dominant process whereby large, deep beds of adsorbent are arranged in vessels to provide sufficient capacity for the gases to be treated. Any number of vessels are used in parallel flow as required to provide the desired capacity and to allow for some of the vessels to be taken out of service when the adsorbent therein has become saturated with the products removed from the gases. When vessels are removed from service to regenerate the adsorbent, it is done either by removing the adsorbent from the vessels for external regeneration and replacing it with fresh adsorbent, or regenerating the adsorbent in place in the vessel by means of special accessories and appurtenances built into the vessels. Devices of this type are limited in size by economic considerations and are capable of processing only limited gas flow rates or gases with low concentrations of material to be removed in the incoming gas stream. For example, the size and number of vessels employed is limited by construction and shipping considerations, and typically are designed to handle roughly 10,000 cubic feet of gas flow per minute. The flow rate capable of being handled by any given fixed bed system is dependent, of course, on vessel size and the number of parallel vessels operational in the process at any given time. Concentration levels in fixed bed systems treating flue gases can range from 1,000 to 3,000 parts per million. An additional reason for this limited capacity of the adsorbent to remove material from the incoming gas flow is that the adsorbent becomes saturated frequently and must be repeatedly taken out of operation to undergo regeneration.
Where moving beds of adsorbent such as activated carbon are employed, the adsorbent is continually fed into an adsorption chamber by a special feeder and, after flowing downward through the chamber by gravity, is continually removed from the chamber by another special feeder and conveyed to other vessels where regeneration is carried out. Because the adsorbent is continually being moved, these moving bed devices suffer the drawback of a measurable loss of adsorbent through attrition. These devices have not yet been proven to be operationally reliable when demonstrated in this country.
U.S. Pat. No. 4,552,726 to Berry, et al. discloses an advanced separation device and method for the continuous treatment of fluid streams. This device is useful for the treatment of plural fluid streams, either by ion exchange or by other solid or fluid contacting operations. However, the equipment does not lend itself to the treatment of large volume gas adsorption separation because it is not amenable to the use of large gas ducts and gas-tight seals which are positioned between the ducts and the adsorption sections of the rotor in such a manner as to allow for minimal pressure losses in the gases as they flow through the adsorption sections and to provide for optimum gas distribution in the adsorbent beds.
There is, for example, a need for method and apparatus to remove sulfur and other harmful constituents from the flue gas of fossil fuel power plants. Due to the large flow rates and elevated temperatures, prior systems are expensive and inefficient.
It is therefore apparent that there is an urgent need for a continuous gas treatment method and apparatus employing an adsorption process which eliminates the abovementioned deficiencies in the prior art and which provides for efficient adsorption over a wide range of concentrations and flow rates in a truly continuous fashion requiring considerably reduced quantities of adsorbent materials than is required by either fixed or moving bed systems.